What race do you play as? If your Terran (you mention depots) then you have two options - first is to scout it then crush it, or if that fails let them build all the way up to your HQ then just lift off and land at your expansion. You may lose a few reactors/techlabs but the amount they'll have wasted on a forge & cannons should put you ahead so long as you can defend your moved command center against whatever they warp into your main. Marauders are extremely good in bunkers against protos.

If a guy refuses to attack, expand behind pressure and just get further ahead. Don't force a fight unless you are positive of victory, just expand like a crazy man. Eventually you'll both have 200 supply but the difference is you should have enough resources and production to rebuild your army much quicker.